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  <div class="question_difficulty">
   难度：Hard
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   <h1 class="question_title">
    145. Binary Tree Postorder Traversal
   </h1>
   <p>
    Given a binary tree, return the
    <em>
     postorder
    </em>
    traversal of its nodes' values.
   </p>
   <p>
    <strong>
     Example:
    </strong>
   </p>
   <pre>
<strong>Input:</strong>&nbsp;<code>[1,null,2,3]</code>
   1
    \
     2
    /
   3

<strong>Output:</strong>&nbsp;<code>[3,2,1]</code>
</pre>
   <p>
    <strong>
     Follow up:
    </strong>
    Recursive solution is trivial, could you do it iteratively?
   </p>
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   <h1 class="question_title">
    145. 二叉树的后序遍历
   </h1>
   <p>
    给定一个二叉树，返回它的
    <em>
     后序&nbsp;
    </em>
    遍历。
   </p>
   <p>
    <strong>
     示例:
    </strong>
   </p>
   <pre><strong>输入:</strong> [1,null,2,3]  
   1
    \
     2
    /
   3 

<strong>输出:</strong> [3,2,1]</pre>
   <p>
    <strong>
     进阶:
    </strong>
    &nbsp;递归算法很简单，你可以通过迭代算法完成吗？
   </p>
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